1. Field of the Invention
The present invention relates generally to the conversion of analog signals to digital form.
2. Description of the Related Art
Analog-to-digital conversion (ADC) is an important scheme in military radio frequency (RF) front-end systems. Although electronic ADCs have been existent for decades, research in photonic ADCs has only gained more focus within the last two decades. This can be attributed to several advantages offered by photonic devices such as; ultra-high bandwidth, compactness, immunity to electro-magnetic interference, low-noise in the system and an efficient semiconductor manufacturing technique for integrated electro-optic devices.
With the increasing demands on performance of modern radar, communication and electronic warfare receivers, a prominent trend is to move the digital signal processing closer to the antenna. Digital microwave receivers and radio frequency (RF) memories are expected to digitize the signals directly at the antenna and eliminate the need for down conversion to intermediate frequencies.
Photonic guided-wave technology has played an important role in the development of advanced analog-to-digital converter (ADC) architectures. Prior art advanced ADCs used a laser and a parallel arrangement of electro-optic Mach-Zehnder interferometers (MZIs) to efficiently couple the RF signal into the optical domain. In the MZI, interference was produced between the phase coherent light waves that have traveled over two different path lengths. The light was input to the MZI through a single mode waveguide and a beam splitter divided the light into two equal beams that travelled through the two waveguides. The split beams then recombined in an output waveguide. By applying the RF voltage to a pair of electrodes along each waveguide, the effective path lengths could be varied. As a result, the RF signal could be used to amplitude modulate the output light.
Normally each MZI in the parallel array symmetrically folded the analog signal with the folding period between MZIs being a successive factor of 2. The folded output amplitude from each detector was quantized with a single comparator (differential amplifier and latch). Together the comparator outputs directly encoded the analog signal in a digital Gray code format. These architectures had great promise due to the large bandwidth available for the MZI (B>45 GHz) and the high pulse repetition frequency (tens of GHz) of mode-locked fiber lasers that could be used for sampling.
One of the major limitations associated with that type of approach was the achievable resolution. For the folding periods to be a successive factor of 2, the length of each electrode must also be doubled which adversely affected the device capacitance and ultimately constrained the feasible resolution. For these 1-bit per interferometer approaches, resolution was limited to less than 4 bits.
A prior art modular preprocessing technique based on the optimum symmetrical number system (OSNS) extended the resolution of the photonic MZI approach beyond 1-bit per interferometer. FIG. 1 is a block diagram illustrating one example of an N=3 interferometer ADC 100 using the optimum symmetrical number system (OSNS) in the prior art. OSNS 100 served as a source for resolution enhancement by using several comparators 108 at the output of each detector 106. That is, instead of using one comparator 108 at each detector 106 output, the OSNS technique analyzed the output of each MZI channel i iε{1,2,3} using one of three sets of comparators 108A, 108B, 108C, where each comparator set was comprised of mi−1 comparators in parallel where mi refers to the OSNS system moduli. FIG. 1 illustrates the prior art implementation for miε{3,4,5}. When the detector 106 output crossed a comparator's matching threshold value, the comparator output changed from a logic zero to a logic one. The mi−1 comparator thresholds were configured to analyze the dynamic range of the detector's output.
Since the MZI transfer function had a (symmetrical) cos2 relationship, the comparator thresholds were arranged in a non-linear fashion. For channel i, the total number of comparators with a logic one at any given time is called a symmetrical residue, xi (or thermometer code); and the combination of all three symmetrical residues from the three sets of comparator systems represents the mapping of the RF antenna voltage into the OSNS domain. A digital representation of the RF signal voltage was then derived within the OSNS-to-binary logic attached at the output of the comparators. By using the OSNS, higher photonic ADC resolutions were feasible using only N=3 MZIs over the 1-bit-per-interferometer architectures described earlier. Unfortunately, using this prior art method, the comparators within each MZI channel must switch states at exactly the same time otherwise an encoding error resulted in the symmetrical residues, which in turn, translated into an error at the output of the OSNS-to-binary logical network.